client/web: add self node cache

Adds a cached self node to the web client Server struct, which will
be used from the web client api to verify that request came from the
node's own machine (i.e. came from the web client frontend). We'll
be using when we switch the web client api over to acting as a proxy
to the localapi, to protect against DNS rebinding attacks.

Updates tailscale/corp#13775

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

tsnet/example/web-client/web-client.go

@@ -5,6 +5,7 @@
 package main
 
 import (
+	"context"
 	"flag"
 	"log"
 	"net/http"
@@ -20,6 +21,7 @@ var (
 
 func main() {
 	flag.Parse()
+	ctx := context.Background()
 
 	s := new(tsnet.Server)
 	defer s.Close()
@@ -30,7 +32,7 @@ func main() {
 	}
 
 	// Serve the Tailscale web client.
-	ws, cleanup := web.NewServer(web.ServerOpts{
+	ws, cleanup := web.NewServer(ctx, web.ServerOpts{
 		DevMode:     *devMode,
 		LocalClient: lc,
 	})